SITE-DIRECTED MUTAGENESIS OF CONSERVED CHARGED RESIDUES IN THE HELICAL REGION OF THE HUMAN C5A RECEPTOR - ARG206 DETERMINES HIGH-AFFINITY BINDING-SITES OF C5A RECEPTOR

The human C5a receptor (C5aR) belongs to the family of G-protein-coupl ed receptors with seven transmembrane helices. This part of the molecu le is thought to contain part of the ligand-binding pocket, specifical ly to bind the C-terminal Arg of human C5a. Guided by sequence similar ity and molecular modelling studies, several residues including polar (Asn119, Thr168, Gln259) as well as all conserved charged amino acids in the upper transmembrane region of the C5aR (Asp37, Asp82, Arg175, A rg206, Asp282) were exchanged by site-directed mutagenesis. Receptor m utants were transiently expressed in COS cells and analyzed for altere d binding behaviour and/or localization at the cell surface by immunof luorescence. For all residues, suitable mutants could be found that ex hibited wild-type affinity towards the ligand, providing evidence agai nst a major contribution of these residues to high-affinity ligand bin ding. Some mutants, however, exhibited a complete (Asp282-->Ala) or pa rtial loss of ligand-binding capacity (Arg175-->Ala, Arg206-->Gln) des pite adequate expression levels on the cell surface. This phenotype wa s further analyzed in the [Gln206]C5aR mutant: quantitative flow cytom etric analysis of epitope-tagged receptor derivatives in 293 cells con firmed an equal level of wild-type and mutant C5aR on the cell surface . Competitive binding curves revealed the presence of only a small pop ulation (<10%) of high-affinity sites (K-d approximate to 2 nM), which was functionally active at 20 nM in the heterologous Xenopus oocyte e xpression system after coexpression of G alpha-16. The number of high- affinity sites of wild-type and [Gln206]C5aR in 293 cells could be up- regulated by coexpression of Gi alpha-2 and down-regulated by GTP[gamm a S]-mediated uncoupling of the C-protein receptor interaction in memb rane preparations. These findings are compatible with a model in which the Arg206 residue located in the upper third of transmembrane helix V determines high-affinity binding in the human C5aR by affecting the intracellular G-protein coupling.